Certain electronic devices are susceptible to damage and accelerated degradation when exposed to chemical constituants found in normal atmospheric air. One of the principal offending chemical compounds but, not the only compound, is water; usually present in the vapor phase. For this reason critical electronic circuits are either incapsulated in isolation material or placed in special containers which are either evacuated to a high vacuum (i.e. vacuum tubes) or pressurized to various levels with dry inert gases (nitrogen, for example), after which the sealed, or isolated, components are placed in a mechanical enclosure, which itself is sealed and pressurized with a similar or different inert gas.
A major problem exists since all mechanical seals have a tendency to leak, and in addition if elastomers are used as the sealing surfaces, gaseous permeation through the elastomic seals will occur. Contamination of the dry inert gas is not only possible, but unavoidable. This is particularly true when the electronic assembly forms part of a space vehicle which is designed to be recovered and recycled (Space Shuttle, for example). Water vapor and other contaminants will permeate through the elastomeric seals at the access parts and/or flanges and eventually will affect the critical components when the contaminants exceed critical threshold levels. Such contamination is subsequently deposited and/or absorbed on internal surfaces and/or electronic components within the enclosure and can be the cause of malfunction or outright failure.
The purging and repressurizing of a sealed electronic enclosure may not dispose of the accumulated water vapor to a high degree. In fact it usually only removes that portion of the water vapor present in the gas; the major portion of the moisture present in the unit would be absorbed within the unit's internal constituents. Complete removal of moisture contained in the device would require thermal bake out under a high vacuum. Accordingly, objective evidence of the readiness of a sealed electronic unit requires a field test apparatus and procedure.
Colometric analysis, which is employed in the novel combination of the invention, is known per se for qualitative and quantitative analysis. In-line detector tubes may be procured commercially or may be specially prepared for detection of specific gases and vapors. One such detector comprises an elongated transparent tube containing a chemical material through which gas may be passed. In the absence of water vapor in the gas, the chemical material is yellow in color. As gas is passed through the tube, however, any water vapor present is absorbed from the input end and on along the tube so that a red color replaces the initial yellow of the chemical material for a distance along the tube proportional to the amount of water absorbed. Such devices are manufactured by Dragerwerk of Lubeck, Germany, and are sometimes called Drager tubes.
U.S. Pat. No. 2,429,694 describes several forms of colometric tubes involving water induced color change in a particulate material through which water-containing gas is passed.
Prior to the invention, no adequate means of performing the desired measurements existed. In fact, conventional gas sampling with subsequent analysis (by mass spectrography, for example), usually compromised the accuracy, or overlooked entirely, the water vapor constituent, because the minute quantities present were absorbed on the sample bottles' interior surfaces. Attempts to reclaim the water vapor constituent usually yielded mixed results because of uncertainty as to the bottles' initial condition.
In the prior art, gas sampling systems and components are variously described in the patent literature.
U.S. Pat. No. 3,903,745 discloses a gas sampling device for collecting and measuring gaseous emission components from flues, but does not address quantitative water vapor detection, although a moisture trap is suggested. Vacuum pumping and a sample collection vessel are shown.
U.S. Pat. No. 4,173,895 is concerned with liquid oxygen sampling and is of interest in that it shows a valve, manifold and pressure gauge arrangement.
U.S. Pat. No. 3,429,186 describes intermittent sampling of a gas flowing in a main, and U.S. Pat. No. 3,539,302 provides a color sensitive material for trace gas detection.
None of the foregoing patents shows or suggests the apparatus or purpose of the invention.
The so-called Drager tube detector which has been incorporated in the novel combination to be described is basically disclosed in U.S. Pat. No. 3,399,973.